A large percentage of the roofing market utilizes asphalt-based products, as for example, modified bituminous rolls, as base substrates which are then topcoated with conventional roof mastics to: improve durability (asphalt degradation); provide energy savings (white coating vs. black asphalt); reduce the fire rating, and improve aesthetics. Such coatings are frequently referred to as "roof mastics" although "mastic topcoat" as used herein include any such thick coatings applied to a substantially horizontal surface, such as for example a roof deck, to a substantially vertical surface such as for example a wall, or to other substrate surfaces. Unfortunately, these mastic topcoats have deficiencies. A roof deck, or other surfaces, by design or because of imperfections, may retain water. This ponded water results in blistering and loss of adhesion leading to failure of the mastic topcoat. An asphalt emulsion tiecoat may be employed to improve the resistance to blistering and loss of adhesion of the mastic coating system. However, the asphalt emulsion tiecoat has a deleterious effect on the resistance to bleed-through discoloration of the mastic coating system. In some cases, highly colored bodies in or on the substrate or, more particularly, in the newly applied tiecoat may migrate into and discolor the mastic topcoat. The aqueous tiecoat of this invention which is composed of a latex-modified asphalt emulsion disposed between a substrate and a mastic topcoat gives the mastic coating system improved resistance to tack and bleed-through discoloration without unduly complicating or hindering the application of the mastic coating system.